1. Field of the Invention
The present invention generally relates to an apparatus for fabricating semiconductor single crystals by a weighing method to control their diameters.
2. Description of the Related Art
At present, most of semiconductor substrates used for fabricating integrated circuits are single crystals of silicon with high purity. However, a substantial percentage of the silicon crystals used by the semiconductor industry are prepared by the Czochralski technique, CZ thereafter. In the CZ crystal-growth process, polycrystalline silicon with high purity is first loaded into a crucible, which is placed in the furnace of a manufacturing apparatus, as a raw material. A main heater around the crucible is utilized to heat the polycrystalline silicon above its melting point. Moreover, a suitably oriented seed crystal is suspended over the crucible in a seed holder, and thereafter is inserted into the melt. It is then slowly withdrawn and rotated in a direction the same as or counter to that of the crucible rotation. Progressive freezing at the solid-liquid interface yields a large, single silicon crystal.
FIG. 4 depicts a conventional apparatus for fabricating semiconductor single crystals in a schematic diagram. Numeral 21 is a wire-rolling drum. Numeral 31 designates a motor for driving the rolling drum 21. Numeral 36 represents a motor for driving the rolling drum 21 and the vacuum container 1 having the driving motor 31 placed thereon to move in a horizontal direction. Thus, pulling the rotating wire 4 rotates grown single crystals. Numeral 37 is a camera, and numeral 38 is a transparent quartz window. Numeral 39 is a seed holder to suspend a seed crystal. Furthermore, numeral 40 represents the melt surface, numeral 41 is a graphite heater for melting polysilicon loaded in the crucible, numeral 42 is a graphite thermal shield, numeral 43 designates a motor for rotating the crucible through a crucible shaft, and numeral 44 designates a motor for moving the crucible up and down. When pulling the single crystals of silicon, the camera 37 takes the image signals of meniscus on the melt-crystal interface and then sends to a camera control unit 45. Then, the image signals inputted to a width-detecting unit 46 allow measurement of the diameter of the grown crystal. Accordingly, the pull-rate mechanism and the heater 41 is slaved to the output of a diameter control means 47 so as to readily control the ingot diameters to a predetermined size. Moreover, numeral 48 is a monitor.
However, in addition to applying the aforementioned wire method to the crystal-pulling apparatus by using an optical system (e.g., the camera 37) to control the diameter of single crystals, a weighing control method is available. According to the weighing control method, a standard crystal weight is utilized to compare with that of the grown single crystal then to adjust the pull-rate and the temperature within the furnace. Accordingly, the single crystal is grown to a diameter approaching to that of the standard crystal. Intuitively, the weighing control method must measure the weight of the grown crystal with high accuracy. Therefore, a rod-shape device, so-called an force-bar, must be suspended at the weight-exerting point. Then, the seed crystal is mounted at the bottom end of the force-bar, and the weight of the grown single crystals can be subjected to the weighing detector. However, if the fabricated crystal ingots are provided with the same length as that of the wire method, the apparatus based on the weighing control method will have a total height 1.5-2 times greater than that of the apparatus by using the wire method. This means the factory equipped with the apparatus utilizing the weighing control method need greater space to dispose said apparatus. Consequently, the manufacture cost using the weighing control method is higher as compared with that of the wire method. Nevertheless, the weighing control method does demonstrate several superb diameter-controlling characteristics, such as excellent reproduction and controllable tails. Therefore, the weighing diameter-controlling method is preferable for fabricating semiconductor crystal over the wire method.
The apparatus for pulling crystals by the wire method is disclosed in Japanese Patent Publication No. 04-89389. While the weighing diameter-controlling method applied to this pulling apparatus, as schematically depicted in FIG. 5, the wire 4 for pulling single crystals is rolled via a guide pulley 27 by the rolling drum 21. Such a structure allows the weighing detector 2 to measure the weight W of the single crystal subjected to the guide pulley 27. However, in the structure as depicted in FIG. 5, the force, that is W+F, subjected to the weighing detector 2 comprises the weight of the crystal accompanying with a pulling force F larger than the crystal weight W. Accordingly, the utilized weight detector should be provided with an ability to tolerate a loading two times heavier than the grown crystal weight. However, a novel weighing detector that can sustain heavy loading results in the problem of increasing manufacturing costs.
According to another way, the pulling wire 4 is pliable, and is formed by twisting of a plurality of metal filaments. The twisted structure makes the wire 4 non-circular in cross-sectional view, but makes the diameter in periodic variation along with its axis. As depicted in FIG. 5, the loading point variation in the crystal weights results from the diameter variation, and torque variation of the rolling drum 21 for pulling the crystals affects the crystal pulling force F, or the detecting value W+F, measured by the weighing detector, producing inaccuracy.
Therefore, the error involved in the weighing signal as measured from the weighing detector can not accurately control the diameter of grown single crystal and high quality crystals are difficult to obtain.